Automatic shifting power transmissions include a plurality of torque-transmitting mechanisms such as friction clutches and brakes. These clutches and brakes are generally fluid-operated mechanisms, which require a fluid pressure control to complete engagement and disengagement of the torque-transmitting mechanism. These mechanisms and their structure are well known in the art, as are many pressure controls for establishing the engagement and disengagement of the torque-transmitting mechanism.
In many automatic shifting transmissions, it is desirable to control the engagement pressure of a torque-transmitting mechanism at an increasing rate or a ramp rate during engagement of the torque-transmitting mechanism and to increase the pressure to a maximum value when the torque transmitting mechanism has been fully engaged. The ramp control of the apply pressure is important in that it controls the frictional engagement at low levels during ratio interchanges when the on-coming torque-transmitting mechanism is being engaged and the off-going torque-transmitting mechanism is being disengaged.
Many of the prior art controls for torque-transmitting mechanisms incorporate variable gain valves wherein a first control rate is used during a portion of the engagement and a second control rate is used during the remainder of the control pressure engagement. Many of these valves incorporate differential areas formed on a valve spool to provide the different gain rates that are required for control of the torque-transmitting mechanism.
Additionally, many of the prior art control mechanisms employ a solenoid signal, which is controlled at pressure levels to provide the required gain at the torque-transmitting mechanism. These solenoids might be a variable bleed pulse-width modulated solenoid (PWM), or a two-stage solenoid, all of which are well known to those skilled in the art. These solenoid pressure controls are generally established by a programmable electronic control module, which contains the necessary information for controlling the torque-transmitting mechanism pressure throughout a shift interchange, or a ratio interchange as well as controlling the pressure after the interchange is completed. Often, the control pressure of the solenoid valve is utilized to provide the full range of torque-transmitting mechanism pressure required for both regulation during ratio interchanges and full engagement.
In many instances, additional pressure gain characteristics are desired to provide differing torque-transmitting mechanism engagement pressures. This is especially true when one torque-transmitting mechanism is employed to effect shifting of more than one gear. In this case, the torque-transmitting mechanism may have multiple pressure requirements for holding full torque in each of the gears. Historically, for each of the pressure gain characteristics, additional system hardware was required. This hardware may include boost valves, regulators, and/or a multiplexed system of solenoids and regulators capable of providing a control pressure to the trim pressure regulator valve.